Laser imaging of biotissue represents an unprecedented recent advance that has revolutionized many areas of biomedicine, especially cell biology and cancer diagnosis. Nevertheless, these techniques are not applicable to broad biomedical tasks requiring visualization of nonfluorescent, low-scattering, and low-refractive absorbing structures. The ultimate aim of this proposal is to overcome the limitations of existing technologies by developing laser photothermal (PT) imaging (PTI) that has high absorption sensitivity for spectral imaging of nonfluorescent living cells in their natural state without standard staining and scanning, including the capability for time-resolved monitoring of the integrated PT response from whole cells. The Specific Aims of this proposal are as follows: Aim 1: Develop a new modification of the laser PT phase-contrast microscope/spectrometer, and estimate its sensitivity and spatial resolution. Aim 2: Study laser-cell interactions with PTI to determine its noninvasive and invasive parameters in living cells. Aim 3: Develop PTI of PT contrast probes, such as gold nanoparticles. Aim 4: Evaluate the ability of the PT technique to visualize conventional fluorescent probes as PT probes. Aim 5: Investigate PT the capability of spectroscopy/microscopy of living cells. The potential advantages of a new PT technology, compared with existing tools, include high-sensitivity, high-resolution imaging of nonfluorescent cells in their native state, without the need for staining and scanning. These techniques will enlarge our understanding of cell microstructure properties in normal and pathologic states and can be applied to cellular cancer diagnostics, guidance of selective cancer therapy with a PT sensitizer, study of drug-cell interactions, control of drug delivery, and some phototherapeutic interventions. The technical realization of PTI is relatively simple, requiring the combination of conventional optical phase-contrast microscopy and the already routinely used nanosecond laser.